A recent trend of energy saving has drawn attention to environmental energy, which exists naturally and does not depend on fossil fuel etc. Examples of widely known environmental energy include electric power generated from solar power or wind power. Environmental energy also includes vibration energy which usually exists and has as large an energy density as the previous environmental energy examples. By efficiently utilizing such usually-existing energy in driving a load, it is possible to save energy. However, in order to stably drive the load, it is preferable to temporarily store generated electric power and then to use the electric power to drive the load.
For example, there is disclosed a technique of (i) temporarily storing electric power generated by an optical power generation element in a storage element and (ii) using the electric power to drive a watch circuit (see Patent Literature 1). According to the technique, when a voltage stored in the storage element becomes less than a voltage of electric power generated by the optical power generation element, the electric power generated by the optical power generation element starts being stored in the storage element. Furthermore, there is disclosed another technique of controlling, in accordance with a voltage stored in a storage battery, start and stop of charging to the storage battery which is mounted on an automobile equipped with a power generation device, such as an alternator, although it is not the one in the field related to the above-described environmental energy (see Patent Literatures 2 and 3). As described above, according to conventional techniques, regardless of whether or not the techniques are the ones in the filed related to the environmental energy, control of charging to a storage device for driving a load is carried out in accordance with a comparison between a stored voltage and a reference voltage.